The fluid transport mechanism located at the corneal endothelium is being studied with the aim of establishing its intimate nature. A powerful new method to measure automatically water volume displacements down to 5 nanoliters has been developed and will be applied to our "in vitro" preparation. The rates of flow will be correlated with the electrical parameters related to the ionic pumps in this layer, such as electrical potential difference, resistance and "short-circuit" current. The influence of varying hydrostatic pressures and osmotic pressure differences on the rate of water flow will be also studied. It is expected that the data will serve to test a model recently proposed for the endothelial transport mechanism.